Continental Drift and Seafloor Spreading

The concept of continental drift, proposed by Alfred Wegener, suggested that continents were once joined together and have since drifted apart. However, Wegener's theory faced skepticism due to his inability to explain the mechanism behind this drifting, leaving the theory somewhat unclear until advancements in the 1960s.

Historical Context

Wegener's theory was not widely accepted until the 1960s despite presenting various evidence.
Advancements in technology during the 1950s and 1960s, such as sonar and submersibles, allowed for better exploration of the ocean floor.

Theory of Seafloor Spreading

The Theory of Seafloor Spreading, hypothesized by Harry Hammond Hess and Robert Dietz in 1960, explains how new oceanic crust is formed and how tectonic plates move.

Mechanism: Hot, less dense material from below the Earth's crust rises at mid-ocean ridges, flows sideways, and creates cracks in the crust. Magma emerges from these cracks, cools, and forms new seafloor.

Key Contributors

Harry Hammond Hess: Proposed the theory of seafloor spreading.
Robert Dietz: Contributed to the understanding of seafloor spreading.
Marie Tharp: Created maps revealing the Mid-Atlantic Ridge, a significant underwater mountain range.

Evidence Supporting Seafloor Spreading

Magnetic Stripes: The Earth's magnetic field reverses, creating new stripes on the ocean floor, providing strong evidence for seafloor spreading.
Molten Material: As tectonic plates separate, molten material from the mantle fills the gaps, forming pillow-shaped rocks on the ocean floor.
Drilling Samples: Analysis of oceanic crust age, sediment fossils, and rock magnetism supports the theory.

Differences Between Theories

Continental Drift Theory posited that continents drifted through static oceans.
Seafloor Spreading demonstrates that the ocean floor is dynamic and a site of tectonic activity.

Mid-Ocean Ridges

Seafloor spreading occurs along mid-ocean ridges, which are large mountain ranges in the ocean. Notable examples include:

Mid-Atlantic Ridge: Separates the Atlantic Ocean into two halves, dividing the North American plate from the Eurasian plate and the South American plate from the African plate.
East Pacific Rise: A mid-ocean rise along the Pacific Ocean floor, separating the Pacific Plate from several other plates including the North American Plate.

Rate of Seafloor Spreading

The rate of seafloor spreading can be calculated using the formula:

Rate\ of\ spreading = \frac{distance\ (cm)}{time}

To convert kilometers to centimeters, use the conversion: 1 km = 100,000 cm.

Sample Problem

If the distance between magnetic stripes is 60 km and they formed 3 million years ago, the rate of seafloor spreading can be calculated as follows:

Given: distance = 60 km, time = 3,000,000 years
Convert distance: 60 km x 100,000 cm = 6,000,000 cm
Rate = 6,000,000 cm / 3,000,000 yr = 2 cm/yr

Additional Calculations

Measure the distance between the center of the spreading center and the border between dark yellow and light green (65 Ma point). If measured as 2 cm, with a scale of 1 cm = 475 km, calculate the real distance moved over the past 65 Ma.
Determine the rate of seafloor spreading for the South Atlantic Ocean Basin, with a distance of 72 km over the past 2 million years.
If a magnetic stripe on the seafloor is 40 km wide and the age of the stripe is 4 million years, calculate the spreading rate.
A rock sample located 120 km away from the mid-ocean ridge is determined to be 4 million years old. Calculate the rate of seafloor spreading in cm/year.

Continental Drift and Seafloor Spreading Explained

Continental Drift and Seafloor Spreading Explained